The invention resides in the field of optical telecommunications networks, and is directed in particular to dispersion measurement in optical networks.
In optical transmission systems, the user traffic is carried by one or more channels traveling between a transmitter and a receiver in optical format. The receiver task is to convert the optical signal back into an electrical format and to extract the user signal. A channel is defined as a carrier wavelength modulated with a user signal. Ideally, a light pulse (representing a digital xe2x80x9c1xe2x80x9d) is a surge of light of a certain power at wavelength xcex0; in fact, the pulse of light has a certain xe2x80x9cwidthxe2x80x9d comprised of a small range of wavelengths about the central wavelength, as shown in FIG. 1A.
The optical fibers used as the transmission medium and most optical components (optical amplifiers, filters) are dispersive, that is, different wavelengths of light travel at slightly different phase velocities Vph=xcfx89/k=c/n(xcex), where c is the vacuum speed of light. The propagation characteristics of each wavelength depend on the effective mode index n(xcex), or the effective propagation parameter k=2xcfx80n(xcex)/xcex. The mode index changes with wavelength, polarization and mode profile, due to material dispersion and due to the waveguide dispersion of the confined mode. The effective mode index n(xcex) shows a non-linear wavelength dependence over an extended spectral domain. As a result, not only the phase velocity, but also the group velocity vg=∂xcfx89/∂k=c/[nxe2x88x92xcex(dn/dxcex)] changes with wavelength. The group velocity is the speed at which non-uniformities in the field intensity, such as an information-carrying modulated pulse train, move through the medium. As an initially short pulse requires some spectral width as dictated by the fundamental property of Fourier transforms, the wavelength-dependence of the group velocity tends to broaden the pulse as it propagates through the fiber, because different spectral components of the pulse travel at different velocities.
This wavelength dependency of the propagation parameter and consequently of the group velocity is termed chromatic dispersion CD, or intra-modal dispersion. FIG. 1B shows a signal xe2x80x98100101xe2x80x99 at the input of an optical link, and FIG. 1C illustrates how the light pulses representing xe2x80x981""sxe2x80x99 widen as the signal travels down the fiber. As a result, the pulse energy of symbols xe2x80x9c1xe2x80x9d spreads into the neighboring symbols xe2x80x9c0xe2x80x9d (ISI or intersymbol interference), so that the receiver could interpret the signal correctly as xe2x80x98100101xe2x80x99, or erroneously as xe2x80x98100111xe2x80x99.
It is evident that reconstructing the user signal from the received optical pulses can pose problems, especially in WDM (wavelength division multiplexed) systems, where a plurality of channels travels over the same link.
The chromatic dispersion parameter D(xcex) is defined as:                               D          ⁡                      (            λ            )                          =                                            ∂              τ                                      ∂              λ                                ·                      1            L                                              EQ        ⁢                  xe2x80x83                ⁢        1            
where ∂xcfx84 is the differential group delay (DGD) of two pulses, i.e. the variation of the travel time (in picoseconds) from the transmitter to the point of measurement, ∂xcex is the differential spectral separation of the two carrier wavelengths (in nanometers) and L is the length of the fiber (in kilometers) over which the dispersion is measured. The target dispersion for a fiber link is defined as:
DT(xcex)=D(xcex)xc2x7Lxe2x80x83xe2x80x83EQ2
For example, for every km of fiber traveled through, two pulses with a 1 nm initial separation of wavelengths will experience a differential group delay of 1 ps, if the dispersion parameter of the fiber is 1 ps/(nm km). Similarly, the two outlying spectral components of a 10 Gb/s pulse with a 0.2 nm spectral width will widen by a whole bit period (100 ps) after some propagation distance, and will then cause bit errors by spreading the pulse energy into the neighboring symbol.
Since the dispersion parameter D is wavelength-dependent, another parameter is defined to characterize dispersion, namely the dispersion slope, given by:
S=∂D/∂xcexxe2x80x83xe2x80x83EQ3
If we assume a linear dispersion dependence on wavelength in some interval xcex94xcex, the slope can be expressed as the ratio of change in the dispersion to the change in the wavelength xcex94D/xcex94xcex calculated with respect to a reference wavelength.
Chromatic dispersion can be corrected, or xe2x80x9ccompensated,xe2x80x9d through the use of specially designed optical components (such as fibers, Bragg gratings) inserted at given locations along the transmission path. For a comprehensive compensation, the total dispersion of the compensating component (which could be packaged e.g. as a dispersion compensating module DCM) must have the same value, but opposite sign to the dispersion of the preceding transmission section, which is obtained if the dispersion is xe2x88x92DT(xcex), namely
DDCMxc2x7LDCM fiber=xe2x88x92Dfiber sectionxc2x7Lfiber section.xe2x80x83xe2x80x83EQ4
With the data rates of optical communication systems increasing through techniques such as dense WDM (DWDM), and network reach increasing through techniques known as ultra long reach (ULR), determination of chromatic dispersion of the fiber and optical components within the systems becomes increasingly important, but also more difficult. Thus, dispersion of each transmission section needs to be determined with as much accuracy as possible to provide accurate compensation, for achieving longer un-regenerated reach and ultimately a less expensive network.
Accurate link dispersion values are particularly useful in wavelength switched network. In these networks, end-to-end physical routes (paths) are dynamically set-up and removed arbitrarily (based on users"" requests), without interruption of the co-propagating traffic. Agility requires accurate knowledge of the link parameters, since matching an end-to-end path to a connection request is based, among other rules, on individual link/path performance. The chances of setting-up a connection along a path increase (and the time-to-service decreases) if the selection process uses accurate path performance parameters, which include end-to-end (link) dispersion.
Fiber cable manufacturers provide chromatic dispersion parameters by wavelength windows for each fiber cable type. Also, most device specifications include CD information. A simple way to determine the total dispersion over a link is to multiply the dispersion coefficient for a certain type of fiber by the fiber length in km and to add to this the specified dispersion of the optical components connected in the respective link.
This method is often used in current point-to-point networks, where each span/link is provisioned based on estimated data, using in addition generous engineering margins to ensure that the span/link will successfully carry the traffic over the specified distance. This is clearly not the best way of using network resources. In addition, in many cases the fiber type is not known; there are no reliable methods to detect the type of the fiber buried in early days of the optical networking. Also, this method assumes a uniform dispersion along the entire fiber cable length, which is not generally true. While this assumption can be used in systems with a small channel-count and short links, it is not satisfactory for wavelength switched DWDM (dense WDM) systems.
A more accurate value of dispersion is evidently obtained by measuring the dispersion. Chromatic dispersion can be determined by performing time domain measurements and frequency domain measurements, as described for example by P. J Dean in xe2x80x9cOptical Fiber Communications, Principles and Practicexe2x80x9d, published in 1985 by Prentice-Hall International, Inc, London, pages 196-202.
However, current dispersion measurement methods cannot be readily used in wavelength switched (agile) networks, for at least the following reasons.
The current networks have a point-to-point architecture that uses span equalization, so that the existing dispersion measuring methods can provide accurate dispersion measurements for a span, which is a relatively short length of fiber (100-150 km) and does not include optical amplifiers.
In agile switched networks, a channel travels for much longer distances in optical format (without regeneration) than in point-to-point networks, passing through a plurality of optical amplifiers and intermediate switching nodes and/or optical add/drop nodes. The current dispersion measuring methods are not suitable for links with multiple optical amplifiers since the amplified spontaneous emission (ASE) accumulated along the link reduces the received signal-to-noise ratio and may introduce severe measurement errors.
As well, the current dispersion measurement methods may not be able to cope with the bandwidth-limiting effects introduced by spectral filters in end-to end links.
Some traditional dispersion measurement methods require bi-directional transmission. An optical amplifier however, contains optical isolators prohibiting bidirectional transmission of probe or reference signals.
Still further, the traditional dispersion measurement methods require special test equipment.
There is a need to provide a method for measuring dispersion of an end-to-end link of an optical network that is reliable, inexpensive and uses standard agile network equipment.
It is an object of the invention to provide a reliable and inexpensive method of measuring dispersion in a wavelength division multiplexed (WDM) network, or/and a dense WDM (DWDM) network.
Accordingly, the invention is directed to a device for measuring dispersion of a link between two switching nodes of an optical network comprising: a phase measuring unit for determining a first phase of a data signal received over the link on a first wavelength xcex1 and a second phase of the data signal received over the link on a second wavelength xcex2; and a dispersion measurement controller for controlling operation of the phase measuring unit, and characterizing the dispersion of the link at a wavelength of interest xcex=(xcex1+xcex2)/2 based on the first and second phases.
According to another aspect, the invention provides a method for characterizing the dispersion of a link of an optical network comprising:
(a) transmitting over the link a data signal over a first test wavelength xcex1 and thereafter over a second test wavelength xcex2; (b) at the output of the link, measuring a first phase of the data signal received on the first wavelength xcex1 and a second phase of the data signal received on the second wavelength xcex2; and (c) characterizing the dispersion of the link at a wavelength of interest xcex=(xcex1+xcex2)/2 based on the difference between the first and second phases . . . .
The measured values for dispersion of each link provided by the invention can be advantageously used to optimize network operation. Knowing the link dispersion allows to accurately select the fixed dispersion compensating modules DCMs and to adjust the tunable DCMs such that the network reach is optimized for each connection. Thus, measuring dispersion at each amplifier site or across multiple fiber spans, allows selection of appropriate fixed span DCMs to accurately compensate the span dispersion. The fixed DCMs may be selected in an open loop; semi-automated closed loop testing can be performed where software selected DCMs are replaced until the target is achieved. Full closed loop adjustment of tunable DCMs (when available) to the link target value can be performed at the switching nodes to accurately compensate for the residual link dispersion. Furthermore, the measured link dispersion helps in selecting the power targets for each wavelength. As well, the measured link dispersion can be stored in a database to provide the dispersion values for post dispersion compensation.
Still further, the dispersion measurement of the invention can be used in agile networks to automate recording of span, link or path dispersion, which in turn can be used by the routing mechanism.
The dispersion measurement unit may use installed network equipment or may be implemented on a dispersion measurement card, that can be temporarily inserted in a shelf at remote sites, or a portable test set may be used.
In addition, the method according to the invention uses standard agile network equipment, resulting in less costly and faster measurements.